1. Field of the Invention
The present invention relates generally to a mobile station radio communication system and, more particularly, to a method for searching cells using PN codes upon power on.
2. Description of the Related Art
In general, a mobile communication system employs a cellular configuration to support many users with limited radio frequency (RF) resources. That is, the service area for radio communication is divided into several small zones or cells, and the RF resources are allocated per cell. FIG. 1 shows an ideal cellular configuration applied to a typical mobile communication system. In a mobile communication system employing such a cellular configuration, it is necessary to detect a location of a mobile station (or cellular terminal) in order to setup a call for which the mobile station is considered to be the call destination. The process of detecting the location of the mobile station is referred to as “location registration”. The location registration can be generally divided into power-on location registration, power-off location registration, time-based location registration, distance-based location registration, parameter changed location registration, command-based location registration, implicit location registration and cell channel location registration.
Power-off location registration is performed, when a user turns off a power switch of the mobile station. However, when this occurs, the power supply to the mobile station is not actually cut off until the power-off location registration is completed. Further, the mobile station does not perform the power-off location registration unless a present location is registered. In a power-off state, if the mobile station is beyond an effective distance at its presently registered location, power-off location registration may not be performed correctly. If the power-off location registration, though uncertain, is performed correctly, it is possible to prevent a mobile switching center from unnecessarily paging the mobile station.
Power-on location registration is performed when the user turns on the power switch of the mobile station. For power-on location registration, the mobile station maintains and manages a power-on/initialization timer and does not attempt connection for the purpose of location registration while the timer is activated. Furthermore, in order to prevent the occurrence of continuous location registrations due to repetitive turning on/off of the power switch of the mobile station for a short time, the location registration procedure starts after a lapse of, for example, 20 seconds.
For location registration, the mobile station determines an adjacent cell to be registered and sends a registration message including location registration information to a base station for the determined adjacent cell over an access channel. This registration message is an actual location registration message, wherein the location registration information includes a location registration type, a slot period index, a mobile station protocol update number, a mobile station class indicator, and an incoming call indicator. Upon receipt of the registration message, the base station sends information about the system and network, and a system parameter message including location registration, handoff (or handover) and power control parameters to the mobile station over a paging channel.
The mobile station performs a PN (Pseudo Noise) code sync operation to search for an adjacent cell, prior to performing the power-on location registration subsequent to turning on the power switch. The PN code sync operation includes two steps; a sync acquisition step and a sync tracking step. In the sync acquisition step, a received PN signal and a locally generated PN signal are adjusted to fall within the range of one chip or less. Further, in the sync tracking step, the above two PN signals are finely adjusted so that phases of the PN signals are matched to each other.
FIG. 3 is a flowchart which illustrates a conventional PN code search procedure in a mobile station. A description will now be provided for the conventional PN code search method with reference to FIGS. 1 and 3.
A mobile station determines, in step 310, whether a power-off request is received from a user. Upon detection of the power-off request, the mobile station performs a power-off location registration in step 312. After completion of the power-off location registration, the mobile station determines, in step 314, whether a power-on request is received from the user. Upon detection of the power-on request, the mobile station searches a cell group where the mobile station is presently located, using a received spread spectrum signal in step 316. After the cell group search, at step 318, the mobile station sequentially searches PN codes for adjacent respective cells in a predetermined order. Upon acquisition of PN code sync for a specific cell in the cell search process, the mobile station performs power-on location registration to register itself at the sync-acquired cell, in step 320.
Now, with reference to FIG. 1, a detailed description will be made as to how the mobile station searches the PN codes for the respective cells. It is assumed herein that the mobile station is synchronized with a cell group to which it presently belongs.
When the mobile station is powered on at a location B of FIG. 1 after being power off at a location A, the mobile station initially searches a PN code (PNKA) for the cell where it was powered off, that is, cell “A”. After the initial PN code search, the mobile station searches the PN codes in predetermined order of, for example, PNKB, PNKC, PNKD, PNKE, PNKF and PNKG. In the PN code search process, the mobile station compares unique PN codes for the respective cells with PN codes included in a sync frame received from the base station, to search for a PN code having the highest receiving power level. That is, detection of a PN code having the highest receiving power means that the mobile station has detected a cell (or base station) from which it will receive a service.
In other words, upon detection of the power-on request, the mobile station first searches a cell group and then sequentially searches the PN codes for the respective cells in the cell group in the predetermined order.
In such a PN code search method, when the user travels to a cell which is last on the list of cells to be searched according to the predetermined order, a call setup time will increase. For example, when the mobile station travels to a cell using the PN code PNKG, the mobile station unnecessarily searches the PN codes PNKB, PNKC, PNKD, PNKE and PNKF, thereby wasting search time on the inapplicable PN codes. This inevitably increases the call setup time.